Abu Ishaq al-Sabay'ai' أبو إسحاق السبيعي عمرو بن عبد الله · Fasting (Kitab Al-Siyam)

That man and the people of his city should not fast as an atonement except that they know (for certain) that the people of a certain city of Muslims had fasted on Sunday. In that case they should keep fast as an atonement.